For the first time since the U.S. housing market collapsed, new condominium high-rises are being built in downtown Boston, Los Angeles, and Seattle as developers wager on a resurgence of the riskiest type of residential property. Construction work crews are also busy in Houston, Miami, New York, San Francisco, and the District of Columbia as financing gradually returns to a sector that lenders have shunned in recent years. Condos are making a comeback following a surge in rental demand that pushed the country's apartment-vacancy rate to the lowest level in at least 10 years.